1. Field of the Invention
The invention relates to an image pickup device and, more particularly, to an image pickup device for controlling a white balance by using an image signal obtained by an image pickup element.
2. Related Background Art
An image pickup device such as video camera, electronic still camera, or the like has white balance adjusting means for adjusting so that a reproduced white color becomes the correct white color. In recent years, a TTL (through the taking lens) adjusting method whereby the white balance is automatically adjusted by an output signal of the image pickup element is frequently used.
FIG. 1 is a block diagram of a conventional image pickup device having the white balance adjusting means of the integrating type (averaging type) TTL method. In FIG. 1, reference numeral 1 denotes a lens; 2 an image pickup element such as a CCD or the like to perform a photoelectric conversion; 3 a luminance signal processing unit to derive a luminance signal Y from an output of the image pickup element 2; 4 a chroma signal processing unit to derive a low frequency component Y.sub.L of the luminance signal, a red signal R, and a blue signal B from the output of the image pickup element 2; 5 and 6 an R gain control unit and a B gain control unit for changing signal levels of the output signals R and B of the chroma signal processing unit 4; 7 and 8 matrix amplifiers to derive color difference signals R-Y and B-Y from the output Y.sub.L of the chroma signal processing unit 4 and outputs R' and B' of the R and B gain control units 5 and 6; and 9 a modulation processing unit for modulating the output Y of the luminance signal processing unit 3 and the outputs R-Y and B-Y of the matrix amplifiers 7 and 8 into specified signals and for enabling those signals to be recorded onto a recording medium or the like (not shown) or an image to be displayed by a monitor. Reference numerals 10 and 11 denote averaging units to average the output signals R-Y and B-Y of the matrix amplifiers 7 and 8 by a few picture planes by integrating them or the like. Reference numeral 32 denotes a control voltage deriving unit for deriving a control voltage suitable for the white balance from outputs of the averaging units 10 and 11 and for controlling the R and B gain control units 5 and 6.
The operation of the above conventional device will now be described hereinbelow with reference to FIG. 1.
An object image formed on the image pickup element 2 is first converted into an electric signal. An output signal of the image pickup element 2 is sent to the luminance signal processing unit 3 and chroma signal processing unit 4. The luminance signal Y is derived from the luminance signal processing unit 3. The low frequency component Y.sub.L of the luminance signal and the red and blue signals R and B are derived from the chroma signal processing unit 4. The signal Y.sub.L is a signal in which the red (R), blue (B), and green (G) components are mixed at a ratio of 0.30 (R): 0.59 (G): 0.11(B). That is, Y.sub.L =0.30R+0.59G+0.11B. The signals R and B among the outputs derived from the chroma signal processing unit 4 are sent to the R and B gain control units 5 and 6, by which their signal levels are changed to adjust the white balance, so that the signals R' and B' are generated. The output Y.sub.L of the chroma signal processing unit 4 and the outputs R' and B' of the R and B gain control units 5 and 6 are sent to the matrix amplifiers 7 and 8, from which the color difference signals R-Y and B-Y are obtained. Wherein, EQU R-Y=0.70R-0.59G-0.11B EQU B-Y=0.89B-0.59G-0.30R
The signals Y, R-Y, and B-Y are sent to the modulation processing unit 9 and are modulated into specified signal formats such as to enable the signals to be recorded onto the recording medium or the like or to enable an image to be displayed onto the monitor and are generated.
The output signals R-Y and B-Y of the matrix amplifiers 7 and 8 are also sent to the averaging units 10 and 11, by which average values of the image signals of one or more picture planes are obtained. The control voltage deriving unit 32 derives control voltages to the R and B gain control units 5 and 6 such that the average signal levels are set to the 0 level (that is, R=B=G), thereby adjusting the white balance.
In the above conventional device, however, in the case of the image pickup of a scene such that a high chroma object occupies almost of the picture plane, there is a problem such that it is difficult to suitably adjust the white balance.
Even in the case of the peak method whereby the color difference signals in the portion where the luminance signal level is equal to or higher than a predetermined value are sampled and the sampled values are used to adjust the white balance without using the average values of the color difference signals as in the conventional device, the signals whose levels are equal to or higher than the predetermined level are not always an achromatic color, so that the white balance cannot be preferably adjusted. Further, a method in which both of the above methods are used in combination is also proposed. Such a method, however, has a problem such that the number of component elements is large and a construction is complicated and a remarkable improvement effect is not obtained.
There is also a problem such that when an ordinary object is photographed, an adequate accuracy is not obtained with respect to the white balance in the case of the peak method.